The Mach number is a measure of the speed of an object relative to the speed of sound in the surrounding medium. It’s represented by the symbol “M” and is calculated by dividing the velocity of the object (such as an aircraft or fluid particle) by the speed of sound in the medium. The Mach number helps classify different regimes of airflow and is crucial in understanding aerodynamics and the behavior of objects moving through fluids.
Mach number Basics Understanding
The Mach number is a way to measure how fast something is moving compared to the speed of sound. It’s represented by the symbol “M” and is calculated by dividing the speed of the object (like an airplane) by the speed of sound in the surrounding air.
Depending on the Mach number of an aircraft or vehicle, we can classify its flight regime into different categories:
- Incompressible: When the Mach number is less than 0.3, approximately. In this case, the change in air density due to the motion of the object is very small and can be ignored.
- Subsonic: Mach numbers between 0.3 and 0.8, approximately. In this regime, the effects of compressibility start to become noticeable. There are two subcategories within this range:
- Low subsonic: Mach numbers between 0.3 and 0.6, approximately. Aircraft flying at these speeds experience relatively low aerodynamic drag.
- High subsonic: Mach numbers between 0.6 and 0.8, approximately. Commercial jet aircraft often operate in this range. They try to avoid reaching transonic speeds (around the speed of sound) because it can lead to increased drag and other aerodynamic challenges.
Understanding the Mach number helps engineers design aircraft that perform efficiently and safely within specific speed ranges.
When the Mach number is between 0.8 and less than 1, we call it transonic. This is a tricky situation because the airflow around the aircraft includes both subsonic and supersonic flows. For example, the airflow over the top of the aircraft’s wings can become supersonic, while the airflow entering through the leading edge remains subsonic.
When the Mach number is equal to or greater than 1, we say the aircraft is flying at supersonic speeds. At Mach 1, the airflow around the aircraft is also at Mach 1, and this is known as the sonic condition.
When the Mach number is much greater than 1—usually more than 5—we call it hypersonic. At these speeds, we start to see effects like kinetic heat and the dissociation of gas molecules.
Mach number Explanation
Understanding the Mach number is important because it tells us how fast an object is moving compared to the speed of sound. The speed of sound is the rate at which pressure waves or disturbances travel through a fluid, like air.
Imagine a scenario where there’s no movement in the air, and the pressure remains constant throughout. But when an airplane flies through the air, it creates a disturbance in the air pressure around it. This disturbance travels outwards like a wave at the speed of sound, informing the surrounding air particles of the approaching object. As a result, the air adjusts its trajectory, pressure, and temperature to accommodate the object’s presence.
When an airplane flies slowly with a Mach number of 0.2, the pressure waves travel faster relative to the plane (Mach 1 versus Mach 0.2), informing nearby air particles smoothly. As the plane’s speed increases but remains below Mach 1, the adjustments in the air become less smooth. However, when the airplane surpasses the speed of sound, like at Mach 2, it outpaces the pressure waves. This causes the waves to stack up in front of the aircraft, creating a sudden pressure difference known as a shock wave. This shock wave spreads outward in a cone shape called a Mach cone, resulting in a sonic boom heard as the aircraft passes by.
At fully supersonic speeds, the shock wave becomes more defined, with only a small subsonic area remaining near the object’s nose. As the Mach number rises, so does the intensity of the shock wave, leading to increased changes in temperature, pressure, and density of the air. At extremely high Mach numbers, the shock wave can even cause gas molecules to ionize and dissociate due to the extreme heat generated.